Backside illumination image sensor and method for reducing dark current of backside illumination image sensor

Abstract

A backside illumination image sensor and a method for reducing a dark current of the backside illumination image sensor. The backside illumination image sensor comprises: a photodiode, a first conductive type isolated layer ( 120 ); a gate structure of a pass transistor, corresponding to the first conductive type isolated layer ( 120 ) and formed on an upper surface of a first conductive type semiconductor substrate ( 100 ), the gate structure ( 130 ) comprising: gate oxide ( 131 ), a gate layer ( 132 ), and a gate sidewall ( 133 ), and the gate structure ( 130 ) correspondingly covering the photodiode; and a floating diffusion zone ( 140 ), formed in the first conductive type semiconductor substrate ( 100 ) and having second conductive type heavy doping. In the backside illumination image sensor, a defect does not easily appear on a surface, right above the photodiode, of the first conductive type semiconductor substrate ( 100 ), so that a dark current is effectively prevented from being produced.

What is claimed is: 
     
       1. A method for reducing dark current of a Back Side Illumination (BSI) image sensor, wherein the BSI image sensor comprises:
 a photodiode formed by forming an area with a second type conductivity in a semiconductor substrate with a first type conductivity, wherein the area with the second type conductivity is defined as an area for collecting photo-generated carriers; 
 an isolation layer with the first type conductivity formed in the semiconductor substrate with the first type conductivity and above the photodiode; 
 a gate structure of a pass transistor formed on a top surface of the semiconductor substrate with the first type conductivity and being corresponding to the isolation layer with the first type conductivity, wherein the gate structure comprises a gate oxide layer, a gate electrode layer and a gate spacer, and the gate structure covers the photodiode correspondingly; and 
 a floating diffusion area formed in the semiconductor substrate with the first type conductivity and being heavily doped with the second type conductivity; 
 and wherein the method comprises: 
 when the semiconductor substrate with the first type conductivity is a P-type semiconductor substrate, applying a voltage ranging from −3.0V to −0.5V to the gate structure, wherein the voltage makes holes accumulate at a portion of the P-type semiconductor substrate corresponding to the gate structure, such that the defects at the interface between the P-type semiconductor substrate and the gate oxide layer are isolated from the N-type area of the photodiode to reduce the dark current; and 
 when the semiconductor substrate with the first type conductivity is a N-type semiconductor substrate, applying a voltage ranging from +0.5V to +3.0V to the gate structure, wherein the voltage makes electrons accumulate at a portion of the N-type semiconductor substrate corresponding to the gate structure, such that the defects at the interface between the N-type semiconductor substrate and the gate oxide layer are isolated from the P-type area of the photodiode to reduce the dark current. 
 
     
     
       2. The method according to  claim 1 , wherein the BSI image sensor further comprises: an isolation ring with the first type conductivity,
 wherein the isolation ring with the first type conductivity is formed in the semiconductor substrate with the first type conductivity, surrounds the photodiode, and is adapted for preventing carrier crosstalk between adjacent photodiodes. 
 
     
     
       3. The method according to  claim 1 , wherein the area with the second type conductivity and the isolation layer with the first type conductivity have self alignment features. 
     
     
       4. The method according to  claim 3 , wherein a photoresist layer is coated on the surface of the semiconductor substrate with the first type conductivity; by using a first mask, a photolithography process comprising exposure and development is performed; and a doping process is performed to form the photodiode and the isolation layer with the first type conductivity. 
     
     
       5. The method according to  claim 3 , wherein a photoresist layer is coated on the surface of the semiconductor substrate with the first type conductivity; by using a first mask, a photolithography process comprising exposure and development is performed, so as to form a groove on the surface of the semiconductor substrate with the first type conductivity; a doping process is performed to form the photodiode in a portion of the semiconductor substrate with the first type conductivity corresponding to a bottom of the groove; sidewalls of the groove is etched to enlarge the groove; and the groove is filled to form an isolation layer with the first type conductivity in a portion of the semiconductor substrate with the first type conductivity corresponding to the groove. 
     
     
       6. The method according to  claim 1 , wherein the BSI image sensor further comprises: a light doped area with the second type conductivity formed in the semiconductor substrate with the first type conductivity; and a light doped area with the first type conductivity formed below the light doped area with the second type conductivity,
 wherein the light doped area with the second type conductivity and the light doped area with the first type conductivity have self alignment features; and 
 wherein at least a portion of the light doped area with the second type conductivity and a portion of the light doped area with the first type conductivity are disposed below the gate structure of the pass transistor, and are in contact with the floating diffusion area. 
 
     
     
       7. The method according to  claim 6 , wherein a photoresist layer is coated on the surface of the semiconductor substrate with the first type conductivity; by using a second mask, a photolithography process comprising exposure and development is performed; and a doping process is performed to form the light doped area with the second type conductivity and the light doped area with the first type conductivity. 
     
     
       8. The method according to  claim 6 , wherein a photoresist layer is coated on the surface of the semiconductor substrate with the first type conductivity; by using a second mask, a photolithography process comprising exposure and development is performed, so as to form a groove on the surface of the semiconductor substrate with the first type conductivity; a doping process is performed to form the light doped area with the second type conductivity in a portion of the semiconductor substrate with the first type conductivity corresponding to a bottom of the groove; sidewalls of the groove is etched to enlarge the groove; and the groove is filled to form the light doped area with the first type conductivity. 
     
     
       9. The method according to  claim 1 , wherein when the first type conductivity is P-type, the second type conductivity is N-type; or when the first type conductivity is N-type, the second type conductivity is P-type.